Hourly temperature and dew-point data were available from the EarthInfo compact-disk17 data base of the National Climatic Data Center. For analysis we used the 24-hour mean value for each day. The air-pollution data were obtained from the data base of the Aerometric Information Retrieval System,18 which is maintained by the Environmental Protection Agency. For population-oriented monitoring variables, we downloaded all available data for PM10, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. For the pollutants measured on an hourly basis, we calculated the 24-hour average. If the levels of pollutants were monitored at multiple locations in a metropolitan area, we averaged the data. To avoid the potential consequences of outlying values, we excluded the highest and lowest 10 percent of values (10 percent trimmed mean) and then averaged the values for each set of monitors, after the value for each monitor had been corrected for its yearly average.